Why would it be zero? Seems like he sold a credit spread. One leg can be exercised which means it gets converted into a large equity position, which will then be offset by the other leg being exercised by the broker when they get around to it.

Since it's a defined risk trade, you can easily sell a large number of contracts which can result in very large stock positions when exercised. It's not an issue, the math works out, but it's something you need to understand before making the trade.